Semi-aromatic polyamides are a class of high-performance polyamides featuring outstanding properties such an excellent resistance to creep and fatigue, good mechanical properties, low moisture absorption, improved dimensional stability, very high strength and stiffness at elevated temperatures, as well as a great resistance to a broad range of chemicals.
Some demanding applications require a high resistance to very high temperatures. Semi-aromatic polyamides are candidates of choice for those applications, typically because of their intrinsic high melt temperatures. However, the thermal stability of those materials still needs to be improved, especially over the long term. When molded compositions are subjected to relatively high temperatures for a prolonged period, such as is the case with objects that serve in automotive under-the-hood applications and in several electric or electronic applications, the compositions generally tend to show a decrease in mechanical properties (such as tensile properties) due to thermal degradation of the polymer. This effect is called heat ageing.
Thermal stabilizers are typically added to polymer compositions to better retain the properties of the polymers upon exposure to elevated temperature. When using a thermal stabilizer, the useful lifetime of the molded material can be extended significantly, depending on the type of material, use conditions and type and amount of heat stabilizer. Examples of heat stabilizers typically used in polyamides are organic stabilizers, like phenolic antioxidants and aromatic amines, and combination of potassium iodide or copper iodide. Unfortunately, semi-aromatic polyamide compositions comprising those stabilizers do not achieve acceptable heat ageing performances that are required in some demanding applications.
It is therefore an object of the present invention to provide semi-aromatic polyamide compositions featuring a very good heat ageing performance while maintaining all the other properties of semi-aromatic polyamides at a good level.
WO 2005/007727 discloses a process for preparing heat stabilized molding compositions comprising melt-mixing of a thermoplastic polymer, a non-metallic inorganic filler and elemental iron having a weight average particle size of at most 450 μm to form a composition wherein the thermoplastic polymer forms a continuous phase and the use of those compositions in applications wherein they are exposed to elevated temperature. WO 2005/007727 claims a remarkable improvement in heat ageing properties, exhibited by a much better retention of the mechanical properties at elevated temperature, in respect of the prior art copper salt/potassium iodide or elemental copper containing compositions. WO 2005/007727 discloses aliphatic polyamides (such as PA 6, PA 6,6, and PA 4,6) compositions comprising an elemental iron thermal stabilizer. It also discloses in its example IV a composition comprising PA 6,6/6T and elemental iron.
WO 2011/051123 relates to thermoplastic molding compounds comprising a polyamide and powdered iron having a particle size of no greater than 10 μm that can be obtained by thermally disintegrating iron pentacarbonyl.
However, the use and handling of powders having very fine particle size should preferably be avoided for health and safety considerations. The risk of inhalation of fine particles raises major health concerns since it has been shown to cause lung cancer, whereas the presence of fine particles in the air presents a number of safety issues, the most dangerous one being an explosion hazard.
The Applicant has surprisingly found out that the addition of PA 6 and/or PA 6,6 to compositions comprising semi-aromatic polyamides and elemental iron having a weight average particle size of at least 10 μm leads to outstanding results regarding the heat ageing performance while maintaining all the other properties of semi-aromatic polyamides at a very good level and without the health and safety risk associated with the prior art.